1. Field of the Invention
The present invention relates to a fuel cell system capable of reducing electric power loss in a power conducting passage for supplying power from a fuel cell to a load.
2. Description of the Related Art
A conventional fuel cell system installable in an electric vehicle is constructed as illustrated in FIG. 5. A fuel 124, such as water and methanol and the like, is supplied into a reformer 128. The reformer 128 produces a fuel gas, such as hydrogen gas, from the fuel 124 through steam reforming reaction with methanol. The fuel gas produced by the reformer 128 and air 130 are supplied into a fuel cell 136. The fuel cell 136 generates an electromotive force through electrochemical reactions. The generated electric power is supplied from the fuel cell 136 to an inverter 144 via a DC/DC converter 138 and a diode 139. The inverter 144 also receives electric power from a battery 140 that is connected in parallel to the DC/DC converter 138. Using the power supplied thereto, the inverter 144 drives a motor 146 to provide driving force for the electric vehicle. The DC/DC converter 138 regulates the voltage output from the fuel cell 136, and applies the regulated voltage to the inverter 144 and the battery 140 in parallel.
A control unit 120 calculates a required output of the inverter 144 (i.e., required electric power) from the amount of depression of an accelerator pedal of the electric vehicle detected by an accelerator pedal position sensor 122. Based on the calculated required output, the control unit 120 controls the inverter 144 so that an electric power corresponding to the required output is supplied to the motor 146 via the inverter 144.
Normally, the fuel cell 136 outputs an amount of power that provides the required output of the inverter 144. However, if the power from the fuel cell 136 alone is not sufficient for the required output of the inverter 144, the control unit 120 sets the output voltage of the battery 140 to a desired value such that a supplement power for the required output is outputted (discharged) from the battery 140 to the inverter 144, by controlling the DC/DC converter 138 to adjust the output voltage of the DC/DC converter 138 in accordance with a state of charge (SOC) of the battery 140 detected by an SOC sensor 142.
If the power from the fuel cell 136 has a surplus after providing the required output of the inverter 144, the control unit 120 controls the DC/DC converter 138 to set the output voltage of the battery 140 to a desired value such that the surplus power is stored (charged) into the battery 140.
That is, the related-art fuel cell system adjusts the output voltage of the DC/DC converter 138 provided between the fuel cell 136 and the parallel circuit of the battery 140 and the inverter 144, so as to set the output voltage of the battery 140 to various desired values, whereby a desired amount of power is outputted (discharged) from or stored (charged) into the battery 140.
Therefore, power from the fuel cell 136 is supplied to the inverter 144, always via the DC/DC converter 138.
However, the power converting efficiency of the DC/DC converter 138 is normally as low as 70-80%. Since power from the fuel cell 136 is supplied via the DC/DC converter 138, the related-art fuel cell system has a problem of a considerable power loss caused by the DC/DC converter 138.